1. Field of the Invention
The present invention relates to Code Division Multiple Access (CDMA) for mobile communications, more particularly to a method and device for estimating carrier frequency offset (CFO) in subscriber terminals in Time Division Synchronous Code Division Multiple Access (TD-SCDMA) system.
2. Description of the Related Art
In the TD-SCDMA system, both base stations and terminals are supposed to send and receive data with nominal carrier frequency. However, there is always some deviation between the actual transmitting and receiving frequency and the nominal frequency because of the technology limitation of the devices. In a base station, the frequency accuracy of an oscillator, in which a carrier frequency offset in a base station required by a TD-SCDMA system is less than 0.05 PPM, may meet to the standard requirement, since limitations subjected by a base station from temperature, size, power consumption, cost and the like are smaller. In a subscriber terminal, however, a carrier frequency offset in a subscriber terminal required by the TD-SCDMA system is less than 0.1 PPM, the frequency accuracy of a selected oscillator may not meet the standard requirement, due to the limitations subjected by the subscriber terminal from temperature, size, power consumption, cost and the like are greater. Consequently, carrier frequency offsets need to be estimated in a subscriber terminal by using a known synchronous downlink pilot signal (SYNC_DL) sent by base stations, and according to the estimated value of carrier frequency offsets to perform automatic frequency control (AFC), so that carrier frequency offsets in the subscriber terminal may meet the standard system requirements.
In a method for estimating carrier frequency offset in prior art, SYNC_DL signals only send by a single base station are utilized in a subscriber terminal to estimate the carrier frequency offset. The base station has one or more antennas to transmit signals with the same carrier frequency. FIG. 1 is a schematic diagram showing a carrier frequency offset estimation system having a plurality of base stations in the prior art.
In the TD-SCDMA system, a dedicated downlink pilot time slot (DwPTS) is set, a base station sends SYNC_DL signals at DwPTS of each frame. Codes by which a SYNC_DL signal is consisted of are used to distinguish adjacent cells using the same carrier frequency, that is, the adjacent cells with the same carrier frequency are distinguished by the SYNC_DL codes. There are 32 different SYNC_DL codes defined in the standard, length of each set of SYNC_DL codes is 64 chips. During the initial search of the cells, the subscriber terminal search the SYNC_DL codes at a specific frequency point used in a cell where it currently expects to resided in to implement the downlink synchronization, and then estimate the carrier frequency offset based on the SYNC_DL codes and adjust the carrier frequency according to the estimated carrier frequency offset value to meet the system requirements.
Provided that the system requires for carrier frequency offset estimation and rough adjusting, adjusted carrier frequency offset in a subscriber terminal is less than 1 kHz, then the subscriber terminal has only received, at a corresponding frequency point, the SYNC_DL signals transmitted by one base station or received SYNC_DL signals of the cell currently expected to resided in are much greater than the SYNC_DL signals of other cells using a same frequency. If the signal power ratio is over 6 dB, it is possible for the adjusted carrier frequency offset of a subscriber terminal to meet to the system requirement. However, if the power difference between the simultaneously received SYNC_DL signals respectively in two cells with the same frequency is small, the SYNC_DL signals transmitted by one cell will interfere those transmitted by another cell. For example, when the power ratios of multiple SYNC_DL signals received by a subscriber terminal at the edge of several cells are within 3 dB, there is about 10-20% probability with the existing methods for carrier frequency offset estimation and adjustment that the carrier frequency offset of the subscriber terminal can't meet the requirement of the system.
In the TD-SCDMA system, because of the short length of a SYNC_DL signal, which is only 64 chips (50 μs), the carrier frequency offset after adjustment permitted by the standard is still greater, for example, less than 1 kHz. Even so, as shown in FIG. 1, when the subscriber terminal simultaneously receives SYNC_DL signals from several base stations, and whose power differences are small such as within 3 dB by using an existing method for carrier frequency offset estimation, which only uses the received SYNC_DL signal in a cell (base station) where it currently expects to reside in, i.e., the highest power SYNC_DL signal, to make a carrier frequency offset estimation, there is about 10-20% probability that the carrier frequency offset in the subscriber terminal after automatic frequency control can't be below 1 kHz. In this case, the initiate search for a cell by a subscriber terminal will not be implemented and probability of success for the initiate search for a cell will be affected.